The invention is in the field of electronic surveillance with particular applicability to sonar-radar installations.
In the prior art various schemes have been employed in sonar and radar apparatus for target tracking. In most sonar systems electro-mechanical apparatus has been employed for beam forming. For example one prior art target tracking arragement uses a servo-actuated beamformer to form two half-beams in identical directions. The outputs of the two half-beams are compared to determine if the target signals they contain coincide in time or if one signal is retarded in time with respect to the other. Usually the comparison of the half-beam outputs is accomplished by phase shift and cross-correlation techniques to develop a tracker error signal. The error signal indicates the magnitude and direction the two parallel half-beams must be moved in order to continually track the moving sonar target. Integrating circuits smooth the error signal to reduce random fluctuations in the indicated target direction. The random tracker error can be made arbitrarily small by making the smoothing time arbitrarily large, but this limits the rate at which the tracker can follow a moving target.
A disadvantage of prior art target tracking designs is that a separate electro-mechanical servo-actuated beamformer must be provided for every target that is tracked. The systems are not compatible with modern digital beamformers which form a large number of digital beams essentially simultaneously using time shared digial circuits.
Our invention overcomes these problems of the prior art by providing a target tracking system having means for digital generation of the tracker error signal and improved means for switching between half-beam pairs when the target moves in direction from one half-beam pair to an adjacent half-beam pair. Another object of our invention is to provide a hysteresis-like characteristic to prevent random switching between half-beam pairs when the target switches over. Another object of the invention is to obtain a better solution to the time quantization problem involved in target tracking by providing digital filter means for changing the time registration of one half-beam with respect to the other in very small increments.